In oil field, water usually is co-produced with crude oil and becomes entrained with the oil. The crude oil must generally be free of water (0.5% or less) before it can be sold and transported in pipelines. Thus, is it necessary to separate the oil from the water to prepare the oil for further processing and sale. The complexity of separating mixtures of water and oil depends upon the physical form of the water. Where the mixture has only "free" water, the water will separate readily from the oil because of the differences in densities of the water and oil. This type of separation presents no problem other than providing a vessel in which water-oil phase separation can occur. However, in many instances, the water can be dispersed throughout the oil in very minute particles, usually with diameters less than 25 microns, forming an "emulsion". An emulsion is very difficult and extremely expensive to separate into water and oil phases.
In these emulsions, the small particles of water are dispersed in the oil in a stable condition for several reasons. First, the area of the interface between oil and water in a stable emulsion is very substantial. For example, the interfacial area of one gallon of water dispersed within oil would range from about 1,000 to 300,000 square feet. Each separate interface must be broken to completely break the emulsion.
Furthermore, in a state of emulsion, the interfacial area of the water in the oil is stabilized against coalescence by two factors. The first factor is the minute size of the dispersed water, which effectively increases the interfacial area.
The second factor is the presence of emulsifying agents or surfactants, such as sodium, potassium, calcium or magnesium salts of organic acids, which are commonly found in producing reservoirs. These agents, which can be found dissolved in the water or the oil of the emulsion, completely coat the interfacial area to form a protective high-viscosity film that surrounds and further stabilizes the dispersed water within the continuous oil phase. This film effectively prevents the coalescence of the dispersed water in the emulsion into separate water and oil phases.
Many types of organic and inorganic materials help stabilize emulsions in this manner, in addition to the aforementioned salts of organic acids. Inorganic solids such as sand, clays, iron sulfide, iron oxide, rust, etc. stabilize water-in-oil emulsions. Organic solids such as asphaltenes and paraffin can also provide stability to emulsions. Gases are trapped in the emulsion, contributing to the stability of the emulsion.
The breaking of emulsions in which water is dispersed in the continuous crude oil phase requires performing certain functions. Initially, the water droplets must migrate towards one another, or flocculate, and the inte-facial protective film surrounding the dispersed water within the emulsion must be broken or destroyed. Then, the particles of water must coalesce into larger droplets of water which can undergo settling through the effects of gravity. Thereafter, the coalesced droplets of water are separated as a water phase from the oil phase.
Breaking of emulsions may be achieved by physical and chemical treatments, application of heat, and electrical methods. Generally, the methods for breaking water-in-oil emulsions employ a combination of these treatments. In many instances, chemical demulsifiers may be employed for assisting in the breaking of the protective film which surrounds the dispersed water. The demulsifiers are added to the emulsion to counter-act the effects of the previously mentioned emulsifying agents or surfactants, including the sodium, potassium, calcium or magnesium salts of organic acids. The demulsifier works best when it is distributed throughout the emulsion so as to be present at all interfaces of the water and oil before the emulsion is processed in a treating facility.
There are a multitude of complex chemical compositions which serve as demulsifiers for breaking water-in-oil emulsions. Surface-active materials have been used successfully as demulsifiers. The demulsifiers usually have a variety of polar components with a preferred solubility ranging from predominantly oil-soluble to predominantly water-soluble. The selection of a demulsifier for breaking a particular emulsion may be based upon actual tests and trials performed on the emulsion. Thus, it is usually not possible to predict the costs associated with the use of chemical demulsifiers, until oil production at the field begins. Usually the type of the demulsifier, and its amount in use, are best established by experimentation directly at the facility used to separate the emulsion into water and oil phases.
Chemicals, for example cationic and anionic surfactants, are commonly added to produced fluids in order to break emulsions in the oil field. Heat and mechanical methods are also commonly used alone, or in conjunction with chemicals to destabilize and break emulsions into oil and water phases. (Millions of dollars are spent each year in chemical treating and heating for control of emulsions). These costs must be added to the already significant costs involved in the production of oil from oil fields. The cost of chemical demulsifiers are one of several costs associated with the production of oil from oil fields that must be seriously considered prior to beginning oil production.
U.S. Pat. No. 3,200,567 discloses the use of ultrasonic treatment in the frequency range of 200 kHz to 400 kHz to break oil-water emulsions in a continuous flow treatment.
U.S. Pat. No. 3,594,312 discloses the use of ultrasonic treatment in the frequency range of 10 to 200 kHz to break oil-in-water emulsions.
Prior art U.S. Pat. Nos. 3,200,567 and 3,594,314, as well as other prior art patents and publications, teach a method of separating an emulsion with ultrasonic energy of a frequency greater than 10 kHz, usually 10 to 200 kHz. The prior art references cause cavitation to break the emulsion.
U.S. Pat. No. 2,257,997 also shows the use of ultrasonics in breaking oil-water emulsions. Sonic waves have been used in a hollow chamber of uniform cross-section to separate components of a fluid mixture as described in U.S. Pat. No. 4,280,823.
U.S. Pat. No. 4,339,247 describes the use of an acoustic transducer attached to a hollow chamber that generates acoustic energy which separates dissolved gases from liquids introduced into the hollow chamber.
U.S. Pat. No. 4,411,814 teaches the use of polyamines and/or polyamine salts as demulsifiers.
U.S. Pat. No. 4,737,265 teaches the use of oxyalkylated alkyl-phenol formaldehyde resins as demulsifiers.
In contrast to the prior art, in the present invention the use of frequencies in the acoustic range, from 500 Hz to 10.0 kHz, the shape of the acoustic chamber and laminar flow pattern of the emulsion through the acoustic chamber, more effectively breaks the emulsion without turbulence due to flow or acoustic cavitation that would possibly re-emulsify the water and oil.
This invention provides an effective and economical method to enhance breaking emulsions containing oil and water with the application of acoustic energy in the frequency range of 500 Hz to 10.0 kHz whether alone or in conjunction with chemical demulsifiers.